Although Serna supports both versions of DITA, DITA 1.0 is obsolescent . We recommend that you create DITA 1.1 documents.
DITA topics are the basic units of DITA content. Each topic deals with a single subject.
DITA maps are documents that collect references to DITA topics to organize them for output to a specific deliverable, including generating navigation files and links to related topics.
For more information, see http://docs.oasis-open.org/dita/v1.1/OS/archspec/ditamarkup.html.